Kinetics of the decomposition of benzoyl peroxide in poly (methyl methacrylate) at high pressures

R. RADO, F. SZÓCS, M. VINKOVICOVÁ, and J . PLAČEK

Polymer Institute, Slovak Academy of Sciences, 809 34 Bratislava

Received 7 November 1972

Investigating the influence of pressure on the rate of the thermal decom­position of benzoyl peroxide in poly (methyl methacrylate) at 100°C, a reac­tion rate decrease, with pressure over the range 2 x 103 — 8 x 103 kp c m - 2

was observed, the found value of the activation volume of this process being 7.5 cm 3 mol - 1 .

Studying the kinetics of the decay of macroradicals generated by a thermal decom­position of benzoyl peroxide in poly (methyl methacrylate) at high pressures [1] a pheno­menon was observed indicating a retardation effect of the pressure not only on the proper interaction of polymer radicals but also on their formation process, i.e. on the very thermal decomposition of the peroxide. This assumption was confirmed by a direct experimental evidence [2]: at 104 kp c m - 2 , the half-life of benzoyl peroxide in such a system has, even at 150°C, the same value (~ 10 min.) as at atmospheric pressure over the range 11U—120°C.

To obtain quantitative data of the retardation effect of the pressure on this reaction, also the extent of its effect was investigated by determining the rate constants of the de­composition process in dependence on pressure over the range 2 X 103— 8 x 103 kp cm - ' 2

at 100°C.

Fi j . 1. Logarithm of the ratio of the initial concentration of benzoyl peroxide ([BP]o = 0.052 mol kg"1) and its con­centration after decomposition ([BP]ť) in poly (methyl methacrylate) at 100°C and 2 x 103 kp c m - 2 in dependence on

time (t).

-5.01 6 P»10'3 [kp cm'2]

Fig. Logarithm of the rate constant of unimolecular decomposition of benzoyl peroxide in poly (methyl methacrylate)

vs. pressure at 100°C.

7 9 6 Chem. zvesti 27 (6) 796-798 (1973)

DECOMPOSITION OF BENZOYL PEROXIDE

F r o m t h e results of t h e decomposit ion of benzoyl peroxide in p o l y ( m e t h y l m e t h -acrylate) a t 100°C a n d 2 x 10 3 kp c n r 2 (Fig. 1) it follows t h a t , t h o u g h a t lower tem­p e r a t u r e (80°C) t h e considered react ion seems t o be of chain c h a r a c t e r [3], u n d e r t h e exper imenta l condit ions a decomposit ion t h a t m i g h t be induced paral le ly w i t h t h e spontaneous process could n o t be observed a n y more . This m a y probab ly be due t o t h e re lat ively low share of t h e chain react ion which, wi th regard t o t h e higher t e m p e r a t u r e a n d lowrer peroxide concentra t ion, is over lapped b y t h ° scat ter ing of t h e m e a s u r e d values.

F o r this reason t h e kinetics of t h e decomposit ion process was character ized b y t h e r a t e c o n s t a n t of a unimolecular react ion, de termined a t t h e pressure of 2 x 10 3 k p c m - 2

from t h e t i m e course of t h e decomposit ion react ion (Fig. 1) a n d a t o ther pressures, o n h r

from paral le l m e a s u r e m e n t s of peroxide loss after t h e elapse of only one react ion period (Table 1). (React ion t imes for individual pressures were chosen so as t o p r e v e n t t h e peroxide loss from exceeding 3 0 % of t h e s tar t ing value.)

Table 1

Concentra t ions of benzoyl peroxide after its decomposi t ion ([BP] ř ) a t 100°C a n d a t different pressures. (The initial concentra t ion of peroxide was 0.052 mol kg~J

in all cases.)

Pressure Reaction time x 1 0 - 3 [BP]j [kp c m - 2 ] [s] [mol k g - 1 ]

1 2.4 0.040 4000 5.4 0.042 6000 14.4 0.034 8000 14.4 0.040

T h e d a t a [ B P ] r are average values calculated from at least three parallel m e a s u r e m e n t s .

Consequent ly a n increase of pressure leads t o a gradual decrease of the decomposit ion r a t e of benzoyl peroxide in poly(methyl m e t h a c r y l a t e ) which, when expressed as t h e logar i thm of t h e r a t e c o n s t a n t vs. pressure (Fig. 2), has a pract ical ly l inear course over t h e invest igated range . T h e act ivat ion volume (zJT/+) of this process calculated from t h e slope of t h e s t ra ight line obtained according t o t h ? equat ion [4]

dink _ / I F *

дР BT

expressing t h e dependence of t h e r a t e c o n s t a n t of a general react ion on pressure ( P ) ,

h a s t h e n a posit ive value A F * = 7.5 c m 3 m o l - 1 . This somewhat higher numerica l va lue

AV*, w h e n c o m p a r e d t o t h e act ivat ion volume indicated for e l e m e n t a r y cleavage pro­

cesses of a single chemical b o n d (up t o 5 c m 3 m o l - 1 [5]) m a y be explained also b y t h e

indirect influence of pressure, n a m e l y via a n increase of viscosity [6] of t h e decomposit ion

m e d i u m . This viscosity increase, which a c c e n t u a t e s t h e reverse recombinat ion of t h e

p r i m a r y radical pairs caused b y t h e cage effect [7], will still increase t h e overall r e t a r d a t i o n

effect of pressure.

Chem. zvesti 27 (6) 796-798 (1973) 7 Q 7

R, RADO, F. SZÖCS, M. VINKOVlCOVÁ, J. PLACEK

Experimental

The benzoyl peroxide used was purified by threefold precipitation from its chloroform solution with methanol, whereas the purity of the product determined analytically reached 99.8%.

Pure methyl methacrylate was obtained by depriving the monomer of the last traces of polymerization impurities according to a usual procedure [8].

Poly (methyl methacrylate) samples containing benzoyl peroxide were prepared by dissolving the required amount of peroxide in monomer and by its polymerization in glass tubes (sealed in nitrogen atmosphere), tempered for 70 hours in a thermostated water bath at 30°C. The peroxide concentration in the final polymer was checked ana­lytically.

From poly (methyl methacrylate) blocks prepared by the above procedure, cylindrical samples (length and diameter 6 mm) were turned out. The proper tempering of samples under pressure was carried out so that they, after having been inserted into a hollow steel cylinder (inner diameter 6 mm), were compressed by a piston to the required pressure and quickly heated to 100°C. The constant tempering conditions were obtained by tem­perature and pressure stabilization (temperature deviations being in the range =Ь2°С and those of pressure ± 100 kp c m - 2 ) . After elapse of the reaction time the press mould was rapidly cooled down and the pressure released. The time necessary for samples heating and cooling was negligible with respect to the tempering period. The content of undecomposed benzoyl peroxide was isolated from the tempered samples — after their disintegration — by an 8-hour extraction with boiling methanol. In the extract obtained, the peroxide was determined by titration of iodine (with a 0.01 N solution of sodium thiosulfate), released from potassium iodide in the medium of glacial acetic acid at laboratory temperature [9].

References

1. Szöcs, F., Plaček, J., and Borsig, E., J. PoUjm. Sei. B9, 753 (1971). 2. Rado, R. and Szócs, F., Vysokomol. Soedin. B14, 564 (1972). 3. Rado, R., Chem. Zvesti 19, 46 (1965). 4. Bamford, C. H. and Tipper, C. F. H., The Theory of Kinetics, p. 316. Elsevier, Amster­

dam, 1969. 5. Neuman, R. С. and Amrich, M. J., J. Amer. Chem. Soc. 94, 2730 (1972). 6. Kuss, E. and Pollmann, P., Z. Phys. Chem. 68, 205 (1969). 7. Ivanchev, S. S., Skubilina, L. V., and Denisov, E. Т., Vysokomol. Soedin. B9, 706

(1967). 8. Burnett, G. M., Mechanism of Polymer Reactions, p. 13. Interscience, New York, 1954. 9. Antonovskii, V. L.,Matalets, B. I., Golysheva, G. P., and Terentiev, V. A., Khimiya

perekisnykh soedinenii. (Chemistry of Peroxidic Compounds.) P. 219. Izd. Akad. Nauk SSSR, Moscow, 1963.

Translated by J. Mynařík

798 Chem. zvesti 27 (6) 796-798 (1973)